Fig. 10.7A).
The information is transmitted to both hemispheres through bilateral connections made by the optic chiasm or through the corpus callosum, or both. When the animal is tested with the left eye open and the right eye closed (see
Fig. 10.7A, center), the task can still be performed because both hemispheres have learned the task. If the optic chiasm is transected before the animal is trained, the result is the same (see
Fig. 10.7B). Information is presumably transferred between the two hemispheres through the corpus callosum. This finding can be confirmed by cutting both the optic chiasm and the corpus callosum before training (see
Fig. 10.7C ). Then the information is not transferred, and each hemisphere must learn the task independently.
A similar experiment was conducted in human patients who had undergone surgical transection of the corpus callosum to prevent the interhemispheric spread of epilepsy (
Fig. 10.8 ). The optic chiasm remained intact, but visual information was directed to one or the other hemisphere by the patient’s fixing vision on the central point of the screen. A picture or name of an object was then flashed to one side of the fixation point, so that visual information about the picture reached only the contralateral hemisphere. An opening beneath the screen allowed the patient to manipulate objects that could not be seen. The objects included those shown in the projected pictures. Normal individuals would be able to locate the correct object with either hand. However, patients with a transected corpus callosum could locate the correct object only with the hand ipsilateral to the projected image (contralateral to the hemisphere that received the visual information). For the hand to explore and recognize the correct object, the visual information must have access to the somatosensory and motor areas of the cortex. With the corpus callosum cut, the visual and motor areas are interconnected only on the same side of the brain.
Another test was to ask the patient to verbally identify what object was seen in the picture. The patient would make a correct verbal response to a picture that was projected to the right of the fixation point because the visual information reached only the left (language-dominant) hemisphere. However, the patient could not verbally identify a picture that was presented to the left hemifield because visual information reached only the right hemisphere.
Similar observations can be made in patients with a transected corpus callosum when different forms of stimuli are used. For example, when such patients are given a verbal command to raise the right arm, they do so without difficulty. The language centers in the left hemisphere send signals to the ipsilateral motor areas, and these signals produce the movement of the right arm. However, these patients cannot respond to a command to raise the left arm. The language areas on the left side cannot influence the motor areas on the right unless the corpus callosum is intact. Somatosensory stimuli applied to the right side of the body can be described by patients with a transected corpus callosum, but these patients cannot describe the same stimuli applied to the left side of the body. Information that reaches the right somatosensory areas of the cortex cannot reach the language centers if the corpus callosum has been cut.
In addition to language, other differences in the functional capabilities of the two hemispheres can be compared by exploring the performance of individuals with a transected corpus callosum. Such patients solve
CHAPTER 10 Integrative Functions of the Nervous System﻿ 219 • Fig. 10.7 Role of the Corpus Callosum in the Interhemispheric Transfer of Visual Information When Learning Involves One Eye.A, Discrimination depends on distinguishing between a cross and a circle. B, Discrimination is between triangles oriented with the apex up or down. C, Discrimination is between vertical and horizontal bars.